I have a 6-frame that I dearly love. I only had a few hives at the time, but in truth, it was also all I could afford. I still can't afford to upgrade yet, but for a one-person operation, it just means I'll be extracting longer. So, if you do have a lot of hives, or plan to, go as big as your funds allow you.
The issue of having to move it is a big one. It's not all about the size, it's the weight of the motor. I didn't think to check that out before hand and I'm not sure how much bigger I can go and be able to do what I need to without help. I'm a one-woman operation here, so you can lift more than me, but it is a consideration.
I'd recommend keeping the 2-frame manual, though (sorry, Tommy!). It will be nice to have the option of only extracting a couple of frames on the fly without having to pull out the big guns.